JP2002172469A - Spot welding method for high strength steel plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a welding method for preventing cracks developing on the welding part and improving the tensile strength of the welding part, in a spot welding for a high strength steel plate. SOLUTION: The spot welding method for the high strength steel plate is characterized in making the nugget diameter 3 equal to 1.2 times or less of the end diameter of an electrode in the spot welding method for the high strength steel plate or the high strength steel plate 1 plated on its surface, wherein the current and the voltage between the electrodes in the spot welding are measured to carry out the calculation using the measured current and the voltage and the value of the substance property, and on the basis of the calculation result, the current is controlled. And the calculation using the measured current and voltage between the electrodes and the value of the substance property is a value calculation for obtaining temperatures for each time in arbitrary positions of the spot welding parts by the value calculation based on a thermal conduction model.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spot welding method for preventing cracks generated in a welded portion and improving the tensile strength of the welded portion in spot welding of a high strength steel sheet.

[0002]

2. Description of the Related Art In recent years, for safety measures of automobiles and for weight reduction for the purpose of lowering fuel consumption and reducing CO ₂ emissions,
There is an increasing need to use high-strength steel sheets for automobile bodies. In the process of assembling a vehicle body, spot welding is mainly used, and it is considered that spot welding is mainly used in welding high-strength steel plates. In spot welding, steel plates are overlapped, sandwiched between two opposed water-cooled copper electrodes, energized while applying pressure to melt the contact parts between the steel plates, and then solidified after energization. This is a welding method for forming a nugget. The quality indicators for spot welds include tensile shear strength (tensile strength when a tensile load is applied in the joint shear direction) and cross tensile strength (tensile strength when a tensile load is applied in the joint peel direction). Is important.

Generally, the higher the strength of a steel sheet, the higher the value of the carbon equivalent Ceq represented by the following equation (1). C
As the value of eq increases, the hardness of the nugget (melted) portion and the heat-affected zone increases, the toughness of the portion decreases, and the crack sensitivity increases. Ceq = C + Si / 30 + Mn / 20 + 2P + 4S (1) (where C, Si, Mn, P, and S indicate the contents (% by mass) of carbon, silicon, manganese, phosphorus, and sulfur in the steel, respectively. )

In spot welding of high-strength steel plates, in order to eliminate the gap between the steel plates in the welded portion by pressing, a higher electrode pressing force than that of a mild steel plate, for example, an electrode pressing force expressed by the following formula (2) is used. In some cases, welding is performed with a pressure higher than the pressure. P = 2.45 × t (kN) (2) where P: electrode pressure (kN), t: plate thickness (mm)

[0005] As described above, when spot welding a high-strength steel sheet in a state where there is a gap between the steel sheets, it is necessary to set the electrode pressing force higher than when spot welding a mild steel sheet. In this case, the welding force (the part in contact with the electrode) is deformed by the electrode pressing force during spot welding. If the deformation of this part is large, cracks easily occur in the welded part due to the synergistic action with the high crack sensitivity of the spot welded part of the high strength steel sheet, and fracture occurs in that part, and the tensile strength of the welded part is reduced. descend. At the same time, the reduction in the thickness of the welded portion due to the electrode pressing force also affects the reduction in tensile strength.

[0006] From the viewpoint of improving the corrosion resistance, a plated material is usually used for the body of an automobile, and the use of a plated material for a high-strength steel plate is being studied. When spot welding a high-strength steel plate with a plated surface,
As shown in FIG. 1, cracks (microcracks) 4 are generated from the surface of the spot welded portion in contact with the electrode 2 and the end of the nugget 3 between the steel plates. This cracking occurs when Zn, which is the main component of plating, enters the spot weld. That is, Zn, which is a main component of plating, or Cu diffused from a copper electrode during spot welding enters the austenite grain boundary from the steel plate surface, or Zn, which is a main component of plating, enters the nugget from between the steel plates. Since the melting point decreases in the portion where the plating component has entered, the solidification is delayed in the portion where the plating has entered during solidification after welding. For this reason, cracks occur in this portion where the melting point is reduced by the shrinkage stress during solidification. Such a phenomenon is conventionally observed even when a steel plate is brazed with brass brazing (Cu-Zn), and is well known as solder embrittlement.

[0007] When a mild steel plate having a plated surface is spot-welded, cracking of the welded portion, which is observed when the above-mentioned high-strength plated steel plate is spot-welded, does not occur. In a high-strength steel sheet, since the carbon equivalent Ceq represented by the above equation (1) is high, the hardness of the nugget (melted) portion and the heat-affected zone increases, and the toughness decreases. As a result, cracks easily occur in portions where plating components enter.

In the case of spot welding a steel plate in a general automobile assembly line, welding conditions that cause scattering during welding are often selected. Scattering is a phenomenon in which the diameter of a nugget generated between steel plates exceeds the electrode tip diameter, and molten metal scatters from between the steel plates. Of course, it is also possible to adjust the nugget diameter by adjusting the welding current and to perform welding without causing scattering. However, even if welding is performed under appropriate conditions so that scattering does not occur at the start of use of the electrode, the electrode tip diameter increases with the number of spots and the welding current density decreases. And eventually no nugget is formed. If welding is performed under the condition that no scattering occurs, such a decrease in the nugget diameter cannot be grasped. On the other hand, when welding is performed under conditions that cause spattering, the nugget diameter becomes smaller due to the increase in the number of hit points, and no spattering occurs. In order to reliably obtain a proper nugget diameter, spot welding is performed under conditions that cause scattering.

[0009]

As described above, in spot welding of a high-strength steel sheet, cracks occur in the spot-welded portion, and problems such as a low tensile strength of the weld portion occur. . Therefore, when a high-strength steel sheet is used, it has been difficult to obtain a high-quality weld, that is, a weld having excellent appearance, high tensile strength, and excellent corrosion resistance.
In order to reduce the weight of a car body, it is necessary to use a high-strength steel sheet with a small thickness, but as long as the above-mentioned problem is encountered, unless a means such as increasing the number of welding points is used, Difficult to use. However, the use of such a means leads to an increase in production time and, consequently, an increase in cost. Also, the degree of freedom in design is considered to be considerably limited.

[0010] It is an object of the present invention to prevent cracks generated at a welded portion in spot welding of a high-strength steel plate and improve the tensile strength of the welded portion.

[0011]

Means for Solving the Problems In spot welding of high-strength steel sheets, the reason why the tensile strength of the welded portion, particularly the cross tensile strength is reduced, is that an excessive current is applied to the welded portion under the conventionally used spot welding conditions. The flow was found to be due to excessive heat input. In the high-strength steel sheet, since the carbon equivalent Ceq is high as described above, the hardness of the nugget portion and the heat-affected zone increases, the toughness decreases, and the crack sensitivity increases. In the excessive heat input area, the deformation of the welded part due to the electrode pressing force increases, and if the part is highly susceptible to cracking, cracks easily occur, and if a tensile load is applied to the welded part in that state, fracture occurs at that part The tensile strength of the weld shows a low value. And, if the welding conditions are adjusted so that the nugget diameter is 1.2 times or less the electrode tip diameter, it is possible to prevent excessive heat input to the welded portion and keep the tensile strength of the welded portion high. Revealed.

Further, as described above, in spot welding on a normal line, scattering occurs during welding. When scattering occurs, a defect occurs at the end of the nugget, and this defect becomes a notch.If the weld is highly susceptible to cracking, cracks can easily occur from this part and the tensile strength may decrease. found. In particular, in the case of the cross tensile test, since the stress concentration around the nugget is intense, these effects become remarkable, and in the case of a high-strength steel sheet, the cross tensile strength shows a low value. If the welding conditions are controlled so that the nugget diameter is 1.2 times or less the electrode tip diameter, the occurrence of spatter during welding is reduced or the spatter does not occur at all. It has been clarified that the occurrence of defects at the ends can be prevented and the cross tensile strength can be kept high.

[0013] Cracks generated in spot welds of high-strength plated steel sheets also occur due to excessive heat input due to excessive current during welding, and the nugget diameter is set to be 1.2 times or less the electrode tip diameter. It has been clarified that by controlling the welding conditions as described above, excessive heat input at the welded portion can be prevented, and cracking at the spot welded portion of the high-strength plated steel sheet can be prevented.

If the welding conditions are controlled so that the nugget diameter is 1.2 times or less the electrode tip diameter, the occurrence of scattering at the time of welding is very small or not at all. Therefore, it cannot be confirmed that the nugget diameter is ensured by generating the scattering as in the related art. In the present invention, by measuring the current at the time of welding and the voltage between the electrodes, it has become possible to secure an appropriate nugget diameter even if the electrode tip diameter changes due to aging of the electrodes.

The present invention has been made based on the above findings, and the gist thereof is as follows. (1) A spot welding method for a high-strength steel sheet, wherein the nugget diameter is 1.2 times or less the electrode tip diameter. (2) A spot welding method for a high-strength steel sheet having a surface plated with a high-strength steel sheet, wherein the nugget diameter is 1.2 times or less the electrode tip diameter. (3) The current at the time of spot welding and the voltage between the electrodes are measured, a calculation is performed using the measured current, the voltage between the electrodes, and the material properties, and the current is controlled based on the calculation result. The spot welding method for a high-strength steel sheet according to (1) or (2). (4) The calculation using the measured current, inter-electrode voltage, and material property values is a numerical calculation for obtaining a temperature at each position at an arbitrary position of the spot welded portion by a numerical calculation based on a heat conduction model. The spot welding method for a high-strength steel sheet according to the above (3), characterized in that:

In the present invention, the nugget refers to a portion that has been melted and solidified in the spot welded portion, and the nugget diameter refers to the maximum diameter parallel to the steel sheet surface. The electrode tip diameter refers to the tip diameter of the electrode in a new electrode. Normally, the tip electrode diameter can be specified in the dome radius (DR) type, the truncated cone (CF) type, the truncated cone radius (CR) type, the eccentricity (EF) type, and the eccentric radius (ER) type specified in JIS C9304. However, since it cannot be specified in the radius (R) type or the dome (D) type, in these cases, it is specified as the contact diameter of the new electrode with the steel plate.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Conventionally, as an index of an appropriate nugget diameter in spot welding, a plate thickness t (mm) of a material to be welded is used.
The relationship has been used. Nugget diameter (mm)
It was considered that about 3.5 to 5.5 ton was appropriate. In this index, the proper nugget diameter does not depend on the electrode tip diameter. Generally, an electrode tip diameter of about 5 √t is often used. On the other hand, in the present invention, in spot welding of a high-strength steel sheet, it is conventionally considered to be inappropriate depending on the electrode tip diameter. It was clarified that even with the nugget diameter, the tensile strength of the weld could be secured in some cases.

When spot welding is performed using an electrode having a predetermined electrode tip diameter, increasing the welding current increases the nugget diameter generated by welding, and conversely, decreasing the welding current decreases the nugget diameter. Therefore, by controlling the welding current, the nugget diameter can be kept at 1.2 times or less the electrode tip diameter as in the present invention.
By setting the nugget diameter to 1.2 times or less of the electrode tip diameter, excessive heat input at the welded portion can be avoided, cracking of the welded portion is prevented in spot welding of high-strength steel sheet, and the tensile strength of the welded portion is reduced. Can be improved.

When the diameter of the nugget is controlled to be 1.1 times or less of the diameter of the electrode tip, cracking of the spot welded portion of the high strength steel sheet can be prevented.
More favorable results are obtained from the viewpoint of improvement in tensile strength.

On the other hand, in order to form a healthy spot weld, the nugget diameter should be at least 0.7 times the electrode tip diameter, and the nugget diameter (mm) should be 3 times in relation to the steel plate thickness t (mm).
It is preferable to be at least 5√t.

In the case of welding a steel plate whose surface is not plated, the contact area between the tip of the welding electrode and the steel plate hardly changes up to about 15,000 hit points. In the welding of steel plates with plated surfaces,
Although the change with time of the tip of the electrode becomes faster, it hardly changes up to about 4000 hit points. Therefore, the present invention can be properly carried out under the welding conditions in which the nugget diameter is optimized using the electrode in the initial stage of use until the electrode tip changes with time.

When the number of continuous spot weldings is further increased, the condition of the contact surface between the electrode tip and the steel plate changes, and the contact area between the electrode and the steel plate increases. The increase in contact area
This results in an increase in the diameter of the welding current, that is, the area of the welding current in the steel sheet. If the energized diameter increases, even if a constant current is applied as the welding current, the current density of the energized part in the steel sheet is reduced, the calorific value is reduced, and the exothermic part changes. Such a change in the heat generation state cannot be caught only by observing the welding current. Therefore, when performing welding with a constant welding current, when the contact area between the electrode and the steel sheet increases with an increase in the number of hit points, the current density in the steel sheet decreases, the calorific value decreases, and the nugget diameter gradually decreases. Phenomenon occurs, and the welding is eventually incomplete. In the past, welding was performed under conditions where spattering occurred during welding, and by monitoring the presence or absence of spattering, the nugget diameter was prevented from becoming too small, but the nugget diameter was set to 1.2 times or less the electrode tip diameter. This technique cannot be used in the present invention.

As described above, the change in the heat generation state cannot be caught only by observing the welding current, but can be caught by observing the inter-electrode voltage together with the energizing current. Become. That is, the inter-electrode resistance R can be obtained from the energizing current and the inter-electrode voltage, and the energizing area S is determined from the inter-electrode resistance, the thickness of the steel sheet, and the specific resistance.
Can be sought. Then, by using these values, it is possible to determine the heat value per unit volume at the energized portion in the steel sheet. Therefore, by measuring the current flowing during welding and the voltage between the electrodes, the contact state between the electrode and the steel plate or between the steel plates can be estimated to some extent.

The above (3) of the present invention is characterized by using the calculation result. The contact area between the electrode and the steel sheet is estimated based on the estimation result, and a welding current capable of setting the nugget diameter to an appropriate nugget diameter in the estimated contact area is selected and the current is adjusted. Even if there is a change, it is possible to keep the nugget diameter constant in accordance with the change over time, and it is possible to stably maintain good welding quality. The appropriate nugget diameter is 1.2 times or less the electrode tip diameter, but the electrode tip diameter means the initial electrode tip diameter before the electrode tip changes with time.

FIG. 2 is a diagram for explaining a spot welding method for a high-strength steel sheet according to the present invention. As shown in FIG. 2, high-strength steel sheets 1 (non-plated material or plated material) are overlapped with each other, and energized while being pressed with copper electrodes 2 to form a molten portion between the steel sheets, and after energizing, cooled. The molten portion is solidified to form a nugget 3. The current and the voltage between the electrodes at the time of welding are measured by the measuring device 5, and necessary calculation is performed by the calculating device 6. The control of the welding current is performed using the control device 7 based on the calculation result.

Next, numerical calculation based on the heat conduction model in the above (4) of the present invention will be described with reference to FIG.

In the heat conduction model, heat generation in the steel sheet,
A model is created for the heat conduction in the steel sheet, the heat conduction from the steel sheet to the electrode, and the heat conduction from the steel sheet to the outside air, and numerical calculations are performed. Regarding the heat conduction from the steel plate to the electrode, the contact area between the steel plate and the electrode can be determined based on the current-carrying area S obtained above, and thus can be incorporated into the heat conduction model. A general model can be used for the heat conduction in the steel sheet and the heat conduction from the steel sheet to the outside air, and the heat generation in the steel sheet depends on the current-carrying area. Since it is required, it is possible to construct a heat conduction model as a whole. By a numerical calculation based on this heat conduction model, the temperature at each time at an arbitrary position of the welded portion can be obtained.

In spot welding, usually 50 Hz
Alternatively, it is performed by applying an alternating current of 60 Hz. During energization, the measuring device 5 is used to measure the energizing current and the inter-electrode voltage in a half cycle of the alternating current, and in the remaining half cycle, the arithmetic device 6 performs a numerical calculation based on the above heat conduction model to calculate the welding portion. The temperature at any position can be determined. With the recent improvement in computer capabilities,
There is no difficulty in performing such a heat conduction calculation within a half cycle of the alternating current.

Using the electrode before the electrode tip changes with time,
Establish welding conditions that can provide the optimum nugget diameter. At the same time, the temperature history at an arbitrary position of the welded portion at the time of welding under these conditions, for example, at the center of the weld is clarified by numerical calculation based on the above-described heat conduction model, and this is set as an appropriate temperature history. Next, at the time of continuous hitting, a numerical calculation based on a heat conduction model is performed by the arithmetic unit 6 using the current measured by the measuring device 5 and the voltage between the electrodes during welding, and the temperature history at that time is compared with the appropriate temperature history. If the temperature is too high, the current is decreased by a command from the control device 7. If the temperature is too low, the current is increased by a command from the control device 7. Adjust the current so that the temperature history is always equal to the appropriate temperature history. Thereby, even if the electrode tip changes with time, the nugget diameter can always be maintained under the proper conditions of the present invention, and the occurrence of cracks in spot welds of high-strength steel sheets can be prevented, and the tensile strength can be improved. Can be kept.

Numerical calculations based on the heat conduction model described above have been conventionally used for the purpose of reducing the occurrence of scattering in spot welding. For example, as disclosed in the Journal of the Japan Welding Society, Vol. 67, No. 4, pp. 45-49. In the above (4) of the present invention, the most significant feature is that the numerical calculation based on the heat conduction model is applied for the purpose of preventing cracking of a welded portion and improving tensile strength in spot welding of a high strength steel plate. .

The high-strength steel sheet used in the present invention is not particularly limited, and has a tensile strength of about 420 to 1200 MPa, a solid solution strengthened type, a precipitation strengthened type (Ti precipitation type, Nb precipitation type), Any type of steel sheet, such as a two-phase structure type (a structure containing martensite in ferrite or a structure containing bainite in ferrite) or a work-induced transformation type (a structure containing residual austenite in ferrite), may be used. . The thickness of the steel sheet generally used in an automobile or the like may be, for example, about 0.4 mm to 4.0 mm. The method for producing the steel sheet may be a hot rolling method or a cold rolling method. Examples of the type of plating for coating the high-strength steel sheet include those containing Zn as a component, for example, Zn, Zn-F
Any type such as e, Zn-Al, Sn-Zn may be used. The spot welding conditions for forming the nugget, that is, the electrode shape, electrode pressing force, welding current, and welding time may be in accordance with general spot welding conditions, but as described above, it is better to increase the electrode pressing force. Since the current at which cracks occur at the spot welds of the high-strength plated steel sheet increases, it is preferable to perform the spot welding with the electrode pressure P set high.

[0032]

(First Embodiment) Using the first invention, spot welding of a high-strength steel plate (bare material) was carried out. As a test material, a non-plated, work-induced transformation-type composite structure high-strength steel sheet (symbol: 780 TBS) shown in Table 1 was used. Section test method for spot welded joints (JIS Z
3139).
Based on the tensile test method (JIS Z3136 Z3137), tensile shear test pieces and cross tensile test pieces were prepared, and spot welding was performed as shown in FIG. At the time of spot welding, as shown in Table 1, the tip diameter is set to 5.
Using CR type electrodes of 5, 6.0, 6.5 mm, the pressure is set in two steps (2.94, 4.75 kN) so that the nugget diameter becomes 1.2 times or less the electrode tip diameter. Conditions (the nugget diameter is 4√t, 5√t, 6√tmm, where t is the plate thickness (mm)) and the nugget diameter exceeds 1.2 times the electrode tip diameter. The conditions were set. The occurrence of cracks was examined from the cross-sectional structure, and the tensile shear strength (TSS) and the cross tensile strength (CTS) were examined from a tensile test. The results are shown in Table 1. In the case where 780TBS was spot-welded, no crack was generated from the surface in any case. Regardless of which electrode was used, spot welding was performed under conditions such that the nugget diameter was 1.2 times or less the electrode tip diameter (conditions 1-3, 7-9,
In Nos. 13 to 15 and 19 to 21), no cracks occurred between the steel sheets, and the tensile shear strength and the cross tensile strength increased with an increase in the nugget diameter. Spot welding in the excessive current range (conditions 4 to 6, 10 to 12, 16 to 18, 22 to 2)
In 4), cracks occurred between the steel sheets, and the tensile shear strength and the cross tensile strength tended to decrease as the current increased. When the pressing force was high, the decrease in the cross tensile strength was remarkable. Was. The results were the same even when a steel type other than the work-induced transformation type composite structure steel sheet was used, or when the experiment was carried out by changing the sheet thickness.

[0033]

[Table 1]

(Second Embodiment) Using the second invention,
Spot welding of a high-strength steel plate (plated material) was performed. As the test materials, two types of work-induced transformation type high-strength composite structure high-strength steel sheets (symbols: 590TGA and 780TGA) shown in Tables 2 and 3, both surfaces of which were subjected to galvannealing, were used. As in the first embodiment, a test piece for observing a cross-sectional structure,
Tensile shear test pieces and cross tension test pieces were prepared, and spot welding was performed as shown in FIG. 590 TGA,
As shown in Table 2, a CR type electrode having a tip diameter of 6.5 mm was used, and pressure was applied in two stages (2.94 kN, 4.75 kN).
N), and the condition that the nugget diameter becomes 1.2 times or less of the electrode tip diameter (the nugget diameter is 4√t, 5√t, 6
条件 tmm, where t is the thickness (m
m)), and spot welding was performed under conditions such that the nugget diameter exceeded 1.2 times the electrode tip diameter. From the cross-sectional structure, the occurrence of cracks was determined, and from the tensile test, the tensile shear strength (TS
S) and the cross tensile strength (CTS) were examined. The results are also shown in Table 2. When spot welding is performed under conditions such that the nugget diameter is 1.2 times or less the electrode tip diameter (conditions 1 to 5).
3, 7 to 9), no cracks occurred between the surface and the steel sheet, and the tensile shear strength (TSS) and the cross tensile strength (C
TS) also increased with an increase in the nugget diameter. However, when spot welding was performed in an excessive current region where the nugget diameter would exceed the nugget diameter (conditions 4 to 6 and 10 to 12), the distance between the surface and the steel sheet increased. Cracking occurred, and the tensile shear strength and the cross tensile strength tended to decrease as the current increased, and when the pressing force was high, the cross tensile strength decreased significantly. Similarly, the 780TGA has a tip diameter of
Using 5.5- and 6.0-mm CR type electrodes, spot welding was performed under the conditions shown in Table 3 with the pressing force set at two stages (2.94 kN, 4.75 kN). Table 3 also shows the results of examining the state of occurrence of cracks, tensile shear strength (TSS), and cross tensile strength (CTS). Regardless of which electrode was used, spot welding was performed under conditions such that the nugget diameter was 1.2 times or less the electrode tip diameter (conditions 13 to 15, 1).
9 to 21, 25 to 27, 31 to 33), no cracks occurred between the surface and the steel plate, and the tensile shear strength (TS
S) and the cross tensile strength (CTS) also increased with an increase in the nugget diameter. However, when spot welding was performed in an excessive current range where the nugget diameter exceeded the range (condition 1).
6 to 18, 22 to 24, 28 to 30, 34 to 36), cracks are generated on the surface and between the steel sheets, and the tensile shear strength and the cross tensile strength tend to decrease as the current increases. When the pressing force was high, the cross tensile strength was significantly reduced. Similar experiments were carried out by changing the steel type or plate thickness other than the work-induced transformation type composite structure steel plate, but the results were the same.

[0035]

[Table 2]

[0036]

[Table 3]

(Third Embodiment) Using the third and fourth aspects of the present invention, spot welding of a high-strength steel plate (bare material, plated material) was performed. As test materials, two types of work-induced transformation type composite structure high-strength steel sheets (symbols: 780 TBS, 780 TGA) shown in Table 4 that are not plated or are galvannealed on both sides ) Was used. First
In the same manner as in the Examples, a test piece for observing a cross-sectional structure, a tensile shear test piece, and a cross tensile test piece were prepared. Using these test pieces, as shown in FIG. 2, while measuring the current during spot welding and the voltage between the electrodes, the nugget diameter was 5.5 mm (5√tmm), which was 1.2 times or less the electrode tip diameter. , But
(t is the plate thickness (mm)) The current was controlled so as to be a constant value, and a continuous spot test of spot welding was performed. In addition, a continuous spot test of spot welding is performed in an excessive current region where the nugget diameter exceeds 1.2 times the electrode tip diameter, and further, a continuous spot test of spot welding with a constant current without controlling the welding current. Was done. Crack occurrence and tensile shear strength (T
SS) and cross tensile strength (CTS) are shown in Table 4. When 780TBS was used as the test material and the current was controlled to keep the nugget diameter at a constant value (5.5 mm) which was 1.2 times or less the electrode tip diameter (conditions 1 to 5), the surface and steel plate No cracks occurred between them, and the tensile shear strength (TSS) and the cruciform tensile strength (CTS) showed stable and high values, but the excessive current range where the nugget diameter would exceed the scattering was generated Spot welding (conditions 6 to
In 10), cracks occurred between the steel sheets, and the tensile shear strength and the cross tensile strength showed low values, and were particularly remarkable in the cross tensile strength. Similarly, when 780TGA was used as a test material and the current was controlled so that the nugget diameter was kept at a constant value (5.5 mm) which was 1.2 times or less the electrode tip diameter (condition 1).
No. 1 to 15), no cracks occurred between the surface and the steel sheet, and the tensile shear strength (TSS) and the cross tensile strength (CT)
S) also showed a stable high value, but when spot welding was performed in an excessive current region where the nugget diameter would exceed the range (conditions 16 to 20), cracks occurred between the surface and the steel sheet. The tensile shear strength and the cross tensile strength showed low values, and were particularly remarkable in the cross tensile strength. In addition, when spot welding was performed at a constant current without controlling the current so that the nugget diameter was constant, no cracks occurred between the surface and the steel sheet, but the nugget diameter increased with the number of spots. The tensile shear strength and the cross tensile strength also decreased.

[0038]

[Table 4]

[0039]

According to the present invention, by adjusting the welding conditions so that the nugget diameter is 1.2 times or less the electrode tip diameter, excessive heat input to the welded portion can be prevented and spot welding of high-strength steel sheets can be performed. The tensile strength of the welded portion can be improved, and the occurrence of cracks in the welded portion in spot welding of a high-strength plated steel sheet can be prevented.

In the present invention, the contact between the tip of the electrode and the steel plate may be measured by measuring the current during welding and the voltage between the electrodes, and further by performing a numerical calculation based on a heat conduction model. Even if the situation changes, an appropriate nugget diameter can be secured.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view for explaining a crack in a welded portion in spot welding and spot welding of a high-strength plated steel sheet.

FIG. 2 is a sectional view for explaining the spot welding method of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 High-strength (plated) steel plate 2 Copper electrode 3 Nugget 4 Crack (micro crack) 5 Measuring device 6 Computing device 7 Control device

Claims (4)

[Claims] 1. A spot welding method for a high-strength steel sheet, wherein the nugget diameter is set to be 1.2 times or less the electrode tip diameter. 2. A spot welding method for a high-strength steel sheet having a surface plated with a nugget diameter of 1.2 times or less the electrode tip diameter. 3. The method according to claim 1, wherein a current at the time of spot welding and a voltage between the electrodes are measured, a calculation is performed using the measured current, a voltage between the electrodes, and a material property value, and the current is controlled based on the calculation result. The spot welding method for a high-strength steel sheet according to claim 1. 4. The calculation using the measured current, inter-electrode voltage, and material property values is performed by numerical calculation based on a heat conduction model to obtain a temperature at an arbitrary position of a spot weld at each time. 4. The spot welding method for a high-strength steel sheet according to claim 3, wherein: